The present invention relates generally to power amplifiers and more specifically to bias circuits for transistors biased in class AB mode.
The use of power amplifiers for transmitting radio frequency signals has many applications, including, but not limited to, radio transmitters, television transmitters, CB radios, microwave links, traffic alert and collision avoidance systems(TCAS) and satellite communications systems.
A critical component of a power amplifier is the voltage bias circuitry for biasing the RF power transistor. For optimal operation, the base emitter voltage of the RF transistor must be precisely controlled over temperature. RF transistors operating class A are easily stabilized over temperature using some type of resistive feedback. Unfortunately, transistors operating class AB are precluded from using feedback to stabilize the RF transistor and more complex bias circuits are necessary.
A typical prior art bias circuit for a class AB transistor is shown in FIG. 1 and operates as follows. The circuit uses a current source 11 which injects current into a reference diode 12 to derive a temperature compensating voltage drop thereacross. An adjustable voltage divider circuit comprising resistor 14A and variable resistor 14B are coupled between diode 12 and transistor 13 for making fine adjustments to the V.sub.BE (base-emitter voltage). Typically a large fraction of the voltage drop across diode 12 is applied to the base of transistor 13.
There are several problems with the prior art bias circuits. One problem is that prior art circuits do not precisely track and compensate for temperature characteristics of RF transistors. Even when a diode is selected which exactly matches the characteristics of the RF transistor, the prior art circuits are unable to provide precise V.sub.BE over a wide temperature range and production variations of the RF transistors. Consequently, the RF transistor does not operate efficiently, may "burn-up" or have a reduced MTBF(mean time between failure).
Another problem with the prior art are the expensive high-power P-N junction devices required to achieve good temperature tracking. The alternative is to use an inexpensive lower-power P-N junction device which provide poor temperature tracking. Thus, applications requiring good temperature tracking are forced to use expensive P-N devices which are large and require high-power (e.g. typically in the range of 100-200 milliamps) to operate.
Yet another problem with the prior art is that either a specific P-N junction device is recommended for use with a target RF transistor or numerous P-N junction devices must be tested to find one which substantially matches the temperature characteristics of the target RF transistor. Further, a small change in the target RF transistor(e.g. a manufacturing change) may require a new search for a matching P-N junction device. This process can be both time consuming and expensive.
Still yet another problem with the prior art is the dependence of the temperature tracking capability of the diode 12 on the fine adjustment of resistors 14. It has been found that adjustments to resistor 14B can alter the temperature tracking capability of diode 12. Thus it is difficult to optimize both the temperature tracking of diode 12 and the setpoint.
No one has yet solved the problem of providing precise V.sub.BE for a class AB transistor over a wide temperature range in a cost effective manner.
Class AB transmitters would be improved by a bias circuit which precisely controls V.sub.BE over temperature and independent of V.sub.BE setpoint adjustments, uses low cost, low power devices, and can use generic P-N junction devices.
Clearly there exists a need for an improved bias circuit for class AB amplifiers.